In conventional microwave ovens, a magnetron energized through a transformer coupling produces microwave energy, i.e., high frequency electromagnetic energy, to cook food within the oven cooking cavity therein. When starting such an oven, a high transient current initially is drawn by the transformer. Such high current surge poses a problem in that it subjects the other components in the energization circuit to possible damage, such as, for example, by welding switch contacts together or unnecessarily burning-out the safety fuses in the circuit. Moreover, the high starting current constitutes a start-up shock to the magnetron which shortens its effective useful life.
It has been found that the high current pulse during starting of the microwave oven can be avoided by using a start switch in series connection with a current limiting or protective resistor in the energization circuit for the magnetron to provide a low starting current therefor, and a relay having a small inherent delayed response coupled across the energization circuit which operates a switch connected in parallel circuit relation with such series circuit for bypassing the protective resistor shortly after the magnetron is energized. In this manner, the current drawn by the magnetron during starting does not reach an excessive level.
One drawback of known energization circuits of this type is that there is a risk of burning the protective resistor in the event the protective resistor is overloaded or subjected to high current for too long a duration, such as, for example, when the start switch is held closed and the relay switch fails to close or is held open by another switch or control.
It has been suggested in U.S. Pat. No. 4,011,427 to use a PTC thermistor, i.e., a thermistor having a positive temperature coefficient, as the protective resistor. On occasion of a fault in the relay coil, the PTC thermistor is heated rapidly, when the start switch is closed, resulting in a rapid increase in resistance and decrease in current through the thermistor. The thermistor is thus not damaged by such fault in the circuit during oven start-up. However, with the energization circuit disclosed in such patent the PTC thermistor only has a useful function during oven start-up. Once the relay operated switch closes and the start switch opens, the thermistor is thereafter bypassed. Accordingly, the thermistor offers no protection to the circuit such as in the event, for example, of a relay failure occurring after the relay operated switch closes. Also such energization circuit may cause an undesirable delay in oven restart even though no malfunction exists in the circuitry, which can be an irritation to the oven user. In addition, the possibility exists of a high current pulse being delivered to the magnetron due to reverse voltage kick emanating from the relay when the oven is shut off such as upon opening of the main supply switch therefor.